1. Field of the invention and Related Art
The present invention relates to a frequency converting apparatus applicable to a transmitter circuit of a digital signal processing system which performs various modulating and filtering operations using digital signal processing techniques in radio communication devices.
Conventionally, a frequency conversion system of this type is constructed, for example, as shown in FIG. 1. The system includes a D/A converters, and a low pass filter 5 which extracts the basic frequency band of the sampled signal, the center frequency which is indicated by fc. The frequency characteristic of the low pass filter 5 is shown in FIG. 3 where fs is the sample frequency.
It is assumed that, for example, when an AM-modulated signal having a carrier frequency of fc and an occupied bandwidth of .DELTA.f is to be extracted, the signal is sampled at the A/D converter 2 using a frequency fs higher than twice (fc+.DELTA.f/2). The digital signal output from the A/D converter 2 is delivered to a digital signal processor 3 to be subjected to predetermined digital AM modulation processing. The digital signal output from the processor 3 is converted back to an analog signal by the D/A converter 4 and then supplied to a low pass filter 5 which has a cut-off frequency of fs/2 to thereby extract the analog AM-modulated signal.
Another method is shown in FIG. 2 in which a signal having a frequency band, for example, a carrier frequency fc, lower than the frequency band of a signal to be extracted is obtained from a low pass filter 7. The output signal from the low pass filter 7 and a sinusoidal signal having a frequency fo from a local oscillator 9 is mixed at a mixer 8 to provide a mixed output via a bandpass filter 10 as a signal having a desired high-frequency band.
According to the former prior art, as the carrier frequency fc increases, the sample frequency fs must be increased accordingly. Therefore, the prior art has the problem that it must use a high speed A/D converter.
According to the latter prior art, the bandpass filter and mixer must additionally be provided, so that the whole scale of the system increases undesirably. Furthermore, the frequencies fs, fc and fo must be appropriately selected such that a spurious radiation produced by the mixing processing does not interfere with the band. Thus there is the problem that the freedom of frequency selection and thus a range of applications are limited.
In addition, if desired frequencies have a band width BW and a central frequency (fc +fo), the frequency characteristic of the bandpass filter 10 is shown in FIG. 4.
As the band width BW to be extacted from the bandpass filter 10 is wide, the lower limit frequency of the pass band comes close to the frequency fo as shown in FIG. 4 and the bandpass filter is required to have a sharp cut-off slope characteristic. Thus the first bandpass filter has the problem that it must be composed of elements of a high quality factor or of a high order.